This invention relates to apparatus for converting the motion of a drive shaft continuously rotating at a constant speed into an oscillating motion.
Such oscillating motions are required by a shuttle swing over an angle of about 180.degree. , a beam and a looper of a sewing machine. Typical prior art apparatus for converting continuous rotation into an oscillating motion includes a four member link mechanism. However, since such link mechanism is constructed such that its crank rod alternately transmits a tension and a compression to the driven link the crank rod is required to have sufficient strength to withstand such tension and compression.
Moreover, with the four member link mechanism it is impossible to vary the length of the driven link at a particular phase so that the driven link moves along a curve resembling a sine curve. More particularly, the driven link is moved along a curve of motion wherein the rotational speed is zero at the upper and lower dead points and reaches a maximum value at phases 90.degree. apart from the upper and lower dead points. Furthermore, as the rotational angles at respective phases are determined automatically with respect to the maximum angle of oscillating it is impossible to provide a constant speed motion at a particular phase. For example, in the case of a driven link that reciprocates and rotates over an angle of 160.degree., assuming that the driven link rotates an angle of 8.degree. for the rotary motion of the drive link of from 200.degree. to 210.degree., it is impossible to change the ratio between the rotary angles of the drive and driven links so that it is impossible for the driven link to move along complicated curves.
Further, in the four member link mechanism it is absolutely necessary that the axes of rotation of respective links should be parallel with each other. As a consequence, the links and connecting pins should be accurately finished thus requiring labour and cost. It is also impossible to change the direction of rotation of the driven link so that it is necessary to use a long crank rod which is heavy and causes vibration.
Further, with the four member link mechanism it is impossible to directly obtain an angle of rotation of the driven link of more than 180.degree. so that when an angle of rotation of more than 180.degree. is desired an amplifying mechanism is required. Various links are connected together by openings and pins and in order to make smooth the operation and to decrease noise it is necessary to finish these openings and pins at high accuracies. Low finish accuracies result in backlash and wear as well as noise.